Mechanistics studies on white matter damage have shown that exposure of axons to hypoxia leads to excessive sodium influx and a consequent inverse functioning of the sodium-calcium exchanger (NCX) that ultimately triggers activation of calcium-mediated cell death cascades. Experimentally, this idea is supported by a large body of experimental observations including that blocking of sodium channels with Tetrodotoxin (TTX) or saxitoxin, blocking of the NCX (with bepridil, benzamil, dichlorobenzamil) or manipulation of the transmembrane sodium gradient by substituting Na+ with Li+ or choline can all protect axons against anoxic injury. Conversely, increasing sodium channel permeability during anoxia with veratridine resulted in greater injury. Under hypoxic conditions, the availability of adenosine triphosphate (ATP) within is axoplasm becomes limited not only due to decrease synthesis but also due to the increased demands from the sodium-potassium adenosine triphosphatase (Na+/K+-ATPase) for extruding exceeding sodium. It has also been shown that in an inflammatory milieu, where nitric oxide (NO) and reactive oxygen species (ROS) are produced by phagocytic cells such as macrophages and microglia, the availability of ATP is diminished by the damage that this mediators can directly cause on mitochondria, particularly on enzymes involved in the synthesis of ATP itself. Through this mechanism, NO donors can exacerbate the axonal damage induced by hypoxia. Indeed, in multiple sclerosis, where a persistent sodium current is hypothesized to overload demyelinated axons and where the synthesis of ATP is affected by NO and reactive oxygen species (ROS) due to the inflammatory nature of this disease, any initial Na+ overload cannot be overcome and creates a vicious cycle, causing reverse functioning of the NCX which in turn activates Ca+2-mediated cell cascades including the increased synthesis of NO, which besides impairing ATP synthesis itself, in addition to triggering axonal degeneration and apoptosis by multiple known mechanisms. This aspect of the pathology of multiple sclerosis is well documented in the literature and has been named virtual hypoxia.
In line with the hypothesis of the association of sodium overload and axonal degeneration in multiple sclerosis are the observations of increased total sodium content in the advanced stage of relapsing-remitting (RR) multiple sclerosis, especially in the normal-appearing brain tissues by using sodium 23 (23Na) magnetic resonance (MR) imaging. Sodium channel blockers such as Phenytoin, Carbamazepine, Flecainide and Lamotrigine are well established drugs and are indicated for different conditions such as epilepsy, neuropathic pain and arrhythmia. All these compound have one feature in common, i.e., they are all state-dependent sodium channel blockers, meaning that they do not affect the normal functioning of sodium channels, but do so particularly in pathological states where higher than normal neuronal firing increases the proportion of channels that are found at any time point in a conformational configuration called inactivated state. This is crucial for the safety of these drugs given that action potentials in the central and peripheral nervous systems (CNS and PNS) and axons are conducted by voltage-gated sodium channels.
All of the above mentioned examples of VGSC blockers have been tested in EAE and have in general been shown to improve clinical scores, ameliorate the axonal loss and demyelination associated with disease and revert the loss in axonal conductivity in the spinal cord of the test animals. Voltage-gated sodium channel blockers also exhibit a protective effect in other disease models including spinal cord injury which is a relevant CNS injury model. Collectively, the body of evidence discussed above was convincing enough to raise interest within the scientific community to test the efficacy of VGSC as neuroprotective agents and Lamotrigine was tested in a randomised, double-blind phase II clinical trial for neuroprotection in secondary progressive MS patients and Lamotrigine treatment reduced the deterioration of the timed 25-foot walk (p=0.02) over 2 years.
Two voltage-gated sodium channel (VGSC) isoforms namely Nav1.2 and Nav1.6 have been shown to be overexpressed in post-morten tissue from multiple sclerosis patients and in different animal models mimicking the disease and that are collectively known as experimental autoimmune encephalomyelitis (EAE). Amongst neurons overexpressing VGSC, those overexpressing Nav1.6 are more frequently co-localized with the degeneration marker β-amyloid precursor protein (APP) than those overexpressing Nav1.2. Indeed, it has long been known that axons selectively expressing Nav1.2 are extremely resistant to anoxic injury. This is likely related to the electrophysiological properties of this channel: Nav1.2 shows greater accumulation of inactivation at high frequencies of stimulation while producing smaller persistent currents in comparison with Nav1.6. On the other hand, Nav1.6 produces large persistent currents that may play a role in triggering reverse functioning of the NCX which can injure demyelinated axons where Nav1.6 and the NCX are co-localized. Collectively, this evidence indicates that the Nav1.6 isoform mediates axonal degeneration in multiple sclerosis.